It is often necessary or desirable to determine the location of a medical probe when performing a diagnostic and/or therapeutic procedure on a patient. As one example, catheters, whether intravascular or extravascular, must typically be navigated through a patient's body in order to locate the operative portion of the catheter adjacent a target tissue region. Traditionally, this has been accomplished using fluoroscopy. In this case, radiopaque elements are located on the distal end of the catheter and fluoroscopically imaged as the catheter is routed through the body. As a result, a two-dimensional image of the catheter, as represented by the illuminated radiopaque elements, thereby allowing the physician to roughly determine the location of the catheter. The use of fluoroscopy in locating catheters is somewhat limited, however, in that the physician is only able to visualize the catheter and surrounding tissues in two dimensions. In addition, fluoroscopy does not image soft tissues, making it difficult for the physician to visualize features of the anatomy as a reference for the navigation. In many procedures, such as cardiac electrophysiology therapy, however, identification of the target tissue region and/or catheter navigation must be made with reference to such features, e.g., the ostium of a pulmonary vein. In this case, fluoroscopy is only used to introduce the catheter within the organ containing the target tissue, e.g., within the heart, and the catheter must be navigated within the heart using other means.
Various types of systems have been developed, or at least conceived, to address this issue. Many of these systems have been focused on the need to efficiently and accurately provide electrophysiological therapy to heart tissue in order to treat cardiac rhythm disturbances. During these procedures, a physician steers a mapping/ablation catheter through a main vein or artery into the interior region of the heart that is to be treated, e.g., using fluoroscopy. The physician makes an electrical map of the interior region of the heart in order to determine the source of the cardiac rhythm disturbances, i.e., the targeted cardiac tissue. The physician then places an ablating element carried on the catheter near the targeted cardiac tissue, and directs energy from the ablating element to ablate the tissue and form a lesion, thereby treating the cardiac disturbance.
Recent advancements in transducer and processing technology have enabled commercially available real-time three-dimensional acoustic imaging of the heart and surrounding vasculature. For example, the SONOS 7500 imaging system, marketed by Philips Medical System located in Bothell, Wash., is an example of one such commercially available system that uses an external device to generate the image. This system provides real-time three-dimensional images of cardiac structures with resolution that is adequate for assisting in catheter navigation and placement during electrophysiology procedures. See, e.g., Lang et al., “A Fantastic Journey: 3D Cardiac Acoustic Goes Live,” Radiology Management, November/December 2002; and “Phillips Prepares to Launch System Upgrade Capable of True Real-Time 3D Echo,” Diagnostic Imaging Scan, The Global Biweekly of Medical Imaging, Vol. 16, No. 18, Sep. 11, 2002, the disclosures of which are hereby expressly incorporated herein by reference.
Although real-time three-dimensional imaging systems, such as the SONOS 7500, provide high resolution images, these types of imaging systems are somewhat limited in the field of electrophysiology therapy, because they do not correlate catheter positions and internal anatomical structures with previously recorded signals and ablation locations.
U.S. Pat. No. 6,353,751 describes a system that can be used to navigate a catheter relative to previously recorded signals and ablation locations. The system includes a basket assembly of mapping electrodes that can be deployed within a chamber of a heart. Once deployed, the basket electrodes can be used to map the heart in order to identify and locate the tissue region to be therapeutically treated, e.g., by identifying the specific basket electrode that is adjacent the tissue region. An ablation catheter can then be introduced into the heart chamber and navigated relative to the basket by wirelessly transmitting electrical signals between the electrodes on the basket assembly and a positioning electrode located on the distal end of a catheter. An ablation electrode on the catheter, which may be the same as the positioning electrode, can then be navigated relative to the basket electrodes, and thus, placed adjacent the target tissue region and operated to create a lesion.
In another navigation system, commercially available as the Realtime Position Management™ (RPM) tracking system and developed by Boston Scientific Corporation, located in San Jose, Calif., a graphical representation of a catheter is displayed in a three-dimensional computer-generated representation of a body tissue, e.g., heart chamber. The three-dimensional representation of the body tissue is produced by mapping the geometry of the inner surface of the body tissue in a three-dimensional coordinate system by placing plurality of acoustic location transducers on an ablation/mapping catheter, and moving the catheter to multiple points on the body tissue while tracking the positions of the catheter within the coordinate system using the location transducers. A graphical anatomical shell is then deformed to conform to the transducer positions as they are acquired. The positions of other catheters to be guided within the body, e.g., a mapping/ablation catheter, is determined by placing acoustic transducers on the these catheters and tracking the positions of the catheters within the three-dimensional coordinate system.
The three-dimensional coordinate system is internally established in the RPM system by placing two reference catheters within known locations of the heart, such as the coronary sinus or right ventricular apex, and transmitting between acoustic transducers located on the reference catheters. The location of the ablation/mapping electrode can then be determined by triangulating each of the location transducers relative to the reference transducers.
U.S. Pat. No. 5,868,673 discloses another means for navigating an ablation probe with tissue in order to facilitate treatment of tumors. In this system, reference acoustic transducers are affixed to the exterior of the patient in order to establish an external three-dimensional coordinate system for locating a location transducer located on the ablation probe as it is moved within the tissue.
In order to provide a less invasive and time-consuming electrophysiological procedure, it may be desirable use a system similar to that described in U.S. Pat. No. 5,868,673, in order to establish an external three-dimensional coordinate system. For example, acoustic reference transducers can be affixed to regions of the patient's body in a manner that allows communication between these reference transducers and location transducers on an ablation/mapping catheter. In this manner, no reference catheters need be introduced into the patient's body, thereby lessening the time of the electrophysiological procedure and any discomfort experienced by the patient.
Unfortunately, the availability of acoustic windows through the patient's chest needed to provide communication channels between the spaced-apart reference transducers and the location transducers located within the heart are somewhat limited. Specifically, anatomical regions, such as the ribs and lungs, have vastly different acoustic characteristics than the surrounding tissue. Thus, as the acoustic energy passes through the patient's body, a majority of the energy will reflect backwards at the interface between the surrounding tissue and the ribs and lungs. Even if a transducer does receive a portion of the acoustic signal that passes through the interface, the velocity of the acoustic signal through the ribs and lungs will be different from the velocity of the acoustic signal through the surrounding tissue. As a result, errors will be injected into the navigation algorithm, which assumes a constant acoustic velocity through soft tissue—about the same acoustic velocity as through water.